One year with a CEM 70

One year with the CEM70 exceptional results.

Tested at 550 mm and 1624 mm. Smooth as butter. Honestly, coming back into astrophotography after a break felt like stepping out of the Stone Age and seeing a TV for the first time.

When I went looking for real information on this mount, the “influencers” weren’t much help. Most of them spend 12 minutes with a teardown setup, then call it a “review.” Not exactly confidence-inspiring.

I compared the CEM design to a SWG. I have my reasons for not going SWG, but since I’ve never owned one, those reasons stay hypothetical.

I’ve been in this game since 2010 mounts, optics, cameras, active optics, even retrofitting a quartz encoder to a CGE Pro with a 5 Hz sampling rate to improve PE correction.

I tuned mounts for fun and tried every type of telescope before life took me away for a few years.

Coming back… there’s a lot of misinformation, wrong terminology, and user error. Trying to get a baseline before buying anything is like decoding an alien transmission.

So, if you don’t like long explanations, here’s the short version:

The CEM70 performs above its price class.

Just buy it. I get smooth sub-arcsecond guiding at 1.1”/px. My average seeing is 1.5–1.8”, with repeatable pointing accuracy.

Out of the box

The CEM70 performed extremely well. Zero drama. First thing that impressed me: the RA lock.

You release the RA clutch, then lock the RA axis with the supplied Allen key.

Brilliant idea it means you don’t stress the gearbox while mounting the OTA.

Shame it isn’t on DEC. I simply kept DEC unlocked and handled it extremely carefully. (I don’t recommend you do this unless you can genuinely manhandle the weight.)

The RA lock clamps both vertically and horizontally. Great for balancing — just remember to remove the key or the belts will express their feelings loudly.

Balance

I build my rigs to be inherently balanced. Rotators shift DEC slightly, but nothing dramatic. True 3D balance depends heavily on gearbox precision, and most consumer mounts have too much backlash for it to matter. A slight imbalance keeps the gears engaged and stops oscillation — this has always been best practice at this price point and below for all manufacturers.

The CEM70 only needs a slight imbalance.

Most mounts benefit from gearbox tuning out of the box, but that’s outside this post.

Short rule: your gearbox should sound linear.

Slewing too fast is the number one thing that kills gears, belts, and motor controllers.

And yes — I hear the oscillation in almost every YouTube video. Totally normal, but also a sign the slew speed is too high.

As a teardown mount or a permanent install, the CEM70 feels properly thought-through. I’ve used it both as a teardown and in the observatory.

iPolar

I used to contort myself into neck-destroying positions in wet grass or snow to look through a polar scope, trying to decipher everything without touching the mount. iPolar ended that nonsense.

My workflow when travelling to a new location:

1. Let GPS set your lat/long before dusk.

2. Use a broom handle to point the mount roughly north.

3. Once the first stars appear, fire up iPolar — it works in lighter conditions than you expect.

4. After initial alignment, then install the OTA.

If my guess is off, I’d rather correct a lightweight mount than a fully loaded one. I’m never far off.

Final PA: SharpCap + ATR26000M + native ToupTek direct-draw driver. Quick, accurate, repeatable. I run it a few times until I see ~00°00’10”. No more “PA offset” tricks — those died with film.

CEM70’s adjustment knobs are smooth with fine threads. People who struggle with DEC locks are overtightening them. If you’re using a tripod, let it settle before trusting any PA readings.

Polar alignment should be done quickly — speed improves accuracy. Iterate until happy.

Guiding

80% of YouTube users run PHD2 way too aggressive.

For me: Predictive PEC + Resist Switch, tight PA, low aggression.

Real-world guiding results (1.1″/px)

Across multiple sessions:

• RA RMS: 0.21–0.31″

• DEC RMS: 0.20–0.26″

• Total RMS: 0.28–0.40″

• RA Osc: 0.14–0.37

• Peak excursions: all under ~0.7″

• Zero lost stars

• No RA/DEC limiting

For an imaging scale of 1.1″/px, everything stays well below the image scale and comfortably inside “excellent guiding” territory. The mount doesn’t hunt, doesn’t fight corrections, and doesn’t wander.

In simple terms:

the CEM70 guides under the image scale and stays there.

Cable management heaven

Built-in USB hub, power hub, and powered USB ports. This eliminates cable torsion and more importantly saves me from cable OCD and the resulting existential crisis. Thank you, iOptron.

Conclusion

After a full year teardown mode, observatory mode, short focal length, long focal length the conclusion is the same:

It performs. It’s consistent. It behaves. It doesn’t fight you.